Round Tower

King Christian IV — who seems to have built half of Copenhagen — completed the Round Tower in 1642 as Europe's oldest functioning astronomical observatory, and the design choice that makes it unique is the absence of stairs. Instead, a 209-metre spiral ramp winds seven and a half times around the hollow core of the tower, wide enough for a horse and carriage to reach the top. This was not decorative whimsy. The ramp was built to transport heavy astronomical instruments and library books to the upper floors, because dragging a 17th-century telescope up a narrow staircase would have been a nightmare.

In 1716, Tsar Peter the Great of Russia visited Copenhagen and reportedly rode his horse all the way up the spiral ramp, while his wife Tsarina Catherine followed in a horse-drawn carriage. The story comes from architect Lauritz de Thurah, writing in 1748, and a patch of the original stone floor from that ride is believed to survive. The tower stands 34.8 metres tall and was the go-to observation platform for Danish astronomers for over three centuries, including Tycho Brahe's successors who used it to map the stars above the city.

The library hall halfway up once housed the entire University of Copenhagen library — 10,000 volumes crammed into a single round room. That collection was destroyed in the Great Fire of Copenhagen in 1728, which consumed a third of the city. The tower survived; the books did not. Today the hall hosts rotating art exhibitions, which feels like an appropriate use for a room that has contained both human knowledge and its destruction.

At the top, the observatory platform offers a 360-degree view of Copenhagen's rooftops and church spires. On a clear day, you can see across the Øresund to Sweden. The observatory is still used by amateur astronomers on winter evenings — making it the oldest observatory in Europe that actually still functions as one.